Microdisplays (.mu.D) are the most recent addition to the family of flat-panel displays. While .mu.Ds are based on a number of different techniques to generate or modulate light, all are based on the use of microfabrication technologies to produce a rectangular array of pixels on a semiconductor (usually silicon) back plane. Examples of .mu.Ds include liquid crystal displays (LCDs), field emission displays (FEDs), and digital micro-mirror displays (DMDs).
Pixels in .mu.Ds can be fabricated to have pitches in the range of 10 .mu.m.times.10 .mu.m to 20 .mu.m.times.20 .mu.m, which results in display array size of approximately 10.24 mm.times.7.68 mm having XGA resolution (1024.times.768 pixels) at 10 .mu.m pixel pitch.
Control, driver and image processing circuits may be embedded into the back plane. When viewed through a suitable magnifying lens, these .mu.D pixel arrays can be designed to appear to the human observer as equivalent to a desktop monitor (e.g., a 15" diagonal monitor) when viewed at distances of approximately twice the diagonal dimension. In these direct-view, magnified applications, .mu.Ds are suitable for use in such applications as portable televisions, compact discs, digital video discs (DVDs), personal digital assistant (PDA) applications and the like.
In contrast, when projected on the back side of a large screen through an optical system and viewed from the front, .mu.Ds may have the potential to produce images that rival conventional front screen projectors using polycrystalline LCDs. In this projection configuration, .mu.Ds may be used in applications such as large screen TVs, multi-user (multi-viewer) computers, multi-media, and home theaters.
However, many design problems have heretofore prevented the realization of this potential. For example, small reflective microdisplays generally have a rather low geometric efficiency, which makes high brightness, projected images difficult to achieve. Small microdisplays also force the use of small size arc lamps or special lamps that provide high luminous output at small source size in order to maintain geometric efficiency.
The construction of a typical reflective LCD .mu.D device is herein described and serves as a starting point for describing tiled .mu.D assemblies. The tiling structures, fabrication methods, and circuits for other reflective or emissive .mu.D displays are essentially the same and are therefore not described separately.
The back plane of a typical .mu.D is formed from a crystalline silicon chip which includes CMOS integrated circuits that have been fabricated using a typical SRAM process as is well known to those skilled in the art. Minimum feature sizes of about 1 .mu.m or less are typical on such chips. No significant difference in the fabrication process, compared to standard CMOS chips, occurs until the application of the upper levels of metal interconnect. Multi-layer Al/SiO.sub.2 metallization is still used, but the topmost metal layer forms a two-dimensional array of rectangular mirrors, each about 10-30 .mu.m on a side with a gap of about 1 .mu.m between each pair. These mirror elements serve as the pixels of the .mu.D, the topmost Al layer being polished to a mirror finish in order to serve as a highly effective optical mirror with a reflectivity generally larger than 80%. The gaps between the mirrors are filled with a dielectric material, typically SiO.sub.2, with a low optical reflectivity. Therefore, the mirror array forms the image plane of the microdisplay.
The ratio of the optically active area of each mirror to the entire area of the mirror plus any optically inactive areas, such as gaps, is called the aperture ratio. Typical .mu.D aperture ratios are on the order of 85%, which is much higher than is possible with direct view transmissive active matrix liquid crystal displays (AMLCDs). The metal layer immediately underneath the mirror surface forms light shields under the gaps that prevent light from reaching the light sensitive CMOS circuitry in the back plane. Each mirror element is connected to a CMOS driver circuit through single or multiple vias that provide the voltage to that particular pixel. The rest of the metal interconnect in/on the back plane is used for conventional addressing (e.g., matrix addressing) of the pixels and for regular circuit functions and services for the CMOS circuitry. The CMOS back plane can also contain some or all of the circuits needed for display addressing (e.g., row and column drivers for matrix addressing), control circuits, and any desired image processing circuits.
Given the CMOS back plane with mirror elements, the .mu.D is assembled as follows. A passivation layer and an LCD alignment layer are applied to the top of the mirror plane. A seal bead with a small fill port is next dispensed around the pixel array in the periphery using screen-printing or a dedicated dispensing system. Separately, a glass cover plate having on its lower side a conductive transparent electrode film (e.g., indium-tin-oxide (ITO)), and possibly another alignment layer for the LC material, is fabricated. Large area microfabrication techniques may be used to make arrays of cover plates, which may be scribed and broken into appropriate sizes. A common cover plate is placed on the seal, aligned to the CMOS back plane mirror array and then bonded to the seal bead. The display is next filled with a suitable liquid crystal material, such as twisted nematic liquid crystal (TN-LC) or ferroelectric liquid crystal (FLC) material. The fill port is then sealed. This liquid crystal fill may also contain spacer particles that are dispensed throughout the fill, unless spacers have optionally been fabricated on top of the mirror array. Alternatively, the well that is formed by the seal can be filled with liquid crystal before the formation of the seal between the back plane and a common cover plate. A polarizer film may be applied on the top surface of the common cover plate or placed elsewhere in the optical system.
Next, the .mu.D component is mounted on an interconnect substrate, usually flex, and connections are made from the edge of the CMOS back plane to the substrate. Finally, the .mu.D component is suitably encapsulated, thus providing environmental protection. Plastic encapsulation is typically used in consumer products. The resulting .mu.D modules produced in this manner are compact, lightweight, and relatively inexpensive.
The optical systems for use with .mu.Ds provides three separate functions: (a) providing light, (b) forming color, and (c) magnifying the image to the desired size. There are several ways to produce color. Most direct view transmissive AMLCDs form color by placing a color triad (e.g., red, green and blue) into each pixel, using white backlight and color filters. This is called the spatial color generation technique. Since this increases the pixel pitch by a factor of three, compared to a monochromatic pixel array, this technique is not preferred in .mu.Ds.
The second mechanism used to produce color is to use a separate display unit for each color and then to combine the colors into a single, final image. This so-called three channel approach is the favored technique in commercial, front-projection displays with polysilicon transmissive LCDs. However, in .mu.D applications, this approach may be acceptable only for large, rear projection systems.
In the third method, the mirror array is illuminated sequentially with different colors, one at a time, thus forming the proper color mix as a time average in the human vision system. This is called the field sequential approach. The sequential field colors can be formed from white light by using a three-color filter on a "color wheel"; or can be formed by three separate color light sources. In some magnified view .mu.Ds compound semiconductor solid state light emitting diodes are modulated to produce the field sequential illumination of the mirror array. Field sequential operation requires a pixel response time that is fast enough to resolve the illumination times. For example, at VGA resolution and a frame rate of 60 Hz, the pixel response time should be on the order of 30 .mu.s.
The magnification of the image can be accomplished using refractive or reflective lens assemblies that are well known and widely utilized in standard optical projection systems.
Consider as an illustration the characteristics of a .mu.D manufactured by Displaytech, a Longmont, Colo. based company. The VGA display has 640.times.480 pixels, a pixel pitch of 13 .mu.m, a pixel spacing of 1 .mu.m, FLC fill, an aperture ratio of 85%, a 85% mirror reflectance, field sequential 15 bit color (32,768 colors), a pixel array size of 8.32 mm.times.6.24 mm, and a 60 mW power consumption. The full display engine uses field sequential illumination from red, blue, and green LEDs, and a single external polarizer. The pixel response time is on the order of .mu.s, fast enough to support field sequential operation.
Although currently available .mu.Ds provide only VGA and SVGA resolutions, much higher resolution devices are anticipated in the future. Table I summarizes characteristics of higher resolution .mu.Ds that may be available. Two pixel pitches, 10 and 30 .mu.m, are given in this table, the 10 .mu.m pitch is representative of field sequential color and the 30 .mu.m pitch of spatial color. The rightmost column specifies the magnification factor for rear projector applications with a 40" screen diagonal. Similar numbers can be generated for magnified .mu.D applications.
TABLE I ______________________________________ Array Magnification Pixel Size to 40" Acronym Resolution Pitch (mm) Diagonal ______________________________________ VGA 640 .times. 480 30 .mu.m 19.2 .times. 14.4 41.8x 10 .mu.m 6.4 .times. 4.8 125x SVGA 800 .times. 600 30 .mu.m 24 .times. 18 33.4x 10 .mu.m 8 .times. 6 100.2x XGA 1024 .times. 768 30 .mu.m 30.1 .times. 23.0 26.6x 10 .mu.m 10.2 .times. 7.7 78.5x SXGA 1280 .times. 1024 30 .mu.m 38.4 .times. 30.7 20.9x 10 .mu.m 12.8 .times. 10.2 62.6x UXGA 1600 .times. 1200 30 .mu.m 48 .times. 36 16.7x 10 .mu.m 16 .times. 12 50.1x TBA 1800 .times. 1440 30 .mu.m 54 .times. 43.2 14.8x 10 .mu.m 18 .times. 14.4 44.5x ______________________________________
It is therefore an object of the invention to provide a tiled, flat-panel display composed of microdisplays.
It is a further object of the invention to provide a tiled, flat-panel display composed of microdisplays and having visually imperceptible seams therebetween.
It is an additional object of the invention to provide a tiled, flat-panel .mu.D display having the .mu.D tiles attached to a common substrate.
It is another object of the invention to provide a tiled, flat-panel .mu.D display having a semiconductor common substrate.
It is a still further object of the invention to embed control circuity in either the semiconductor back plane of the .mu.Ds or in the semiconductor common substrate, or both.
It is an additional object of the invention to provide a common substrate which is thermally and mechanically matched to the thermal and mechanical characteristics of the individual .mu.D tiles.
It is yet another object of the invention to provide cooling structures as part of the tiled, flat-panel .mu.D structure to maintain the operating temperature of the display.
It is a still further object of the invention to provide heat-generating means in the display which, when coupled to appropriate temperature sensors and control circuitry, can dynamically maintain an essentially fixed temperature in the tiled, flat-panel .mu.D assembly.